In order to improve communication quality and speed further to cope with an abrupt increase in mobile data traffic in recent years, standardization of a carrier aggregation (CA) function of allowing a radio base station (eNode B (eNB)) and a radio terminal (user equipment (UE)) to communicate with each other using a plurality of cells has been discussed in 3GPP LTE (Long Term Evolution). The cells that a UE can use in the CA are limited to cells of one eNB (i.e., cells served by one eNB).
The cells used by the UE are classified into a primary cell (PCell) which has already been used as a serving cell at the start of CA and a secondary cell (SCell) which is used additionally or subordinately. Non-Access Stratum (NAS) mobility information, security information (security input), and the like are sent and received through the PCell during radio connection (re)-establishment (RRC connection Establishment/Re-establishment) (see Non-Patent Literature 1). A DL carrier corresponding to the PCell is a DL primary component carrier (DL PCC) and its corresponding UL carrier is an UL PCC. Similarly, a DL carrier corresponding to the SCell is a DL secondary component carrier (DL SCC) and its corresponding UL carrier is an UL SCC.
In order to enable a UE to use a SCell in CA, an eNB notifies the UE of configuration information (SCell configuration) of candidate cell(s) for the SCell and activates a cell that is actually used by the UE. A procedure of addition, release, activation, and deactivation of a SCell will be described with reference to FIG. 17.
In step S1, a UE establishes RRC connection in Cell1 of a NB (RRC connection establishment). The Cell1 is the PCell. In step S2, the eNB transmits, to the UE, configuration information including a list of cells (in this example, Cell2 and Cell3) to be added as SCells (RRC Connection Reconfiguration including SCell addition list). In step S3, the UE adds the Cell2 and Cell3 as SCells (SCell=Cell2, 3 addition). At this time point, the UE cannot transmit or receive data in the Cell2 and Cell3. In step S4, the eNB transmits an instruction to activate the Cell2 as a SCell (SCell=Cell2 Activation Control). In step S5, the UE activates the Cell2 (SCell=Cell2 activation). In this way, in step S6, the UE and the eNB transmit and receive data using the Cell1 and Cell2 (Carrier Aggregation on Cell1 and Cell2).
At a certain time point (step S7), the eNB determines to use the Cell3 as a SCell instead of the Cell2 and transmits an instruction to deactivate the Cell2 and an instruction to activate the Cell3 to the UE (SCell=Cell2 Deactivation Control and SCell=Cell3 Activation Control). In steps S8 and S9, the UE deactivates the Cell2 (SCell=Cell2 deactivation) and activates the Cell3 (SCell=Cell3 activation). In this way, in step S10, the UE and the eNB transmit and receive data using the Cell1 and Cell3 (Carrier Aggregation on Cell1 and Cell3).
At a certain time point (step S11), the eNB determines that Carrier Aggregation is not necessary and transmits an instruction to release SCells to the UE (RRC Connection Reconfiguration including SCell release list). In step S12, the UE releases the Cell2 and Cell3 from SCells (SCell=Cell2, 3 release).
Moreover, a concept of Inter-eNB CA of aggregating a plurality of cells served by different eNBs has been proposed (Non-Patent Literature 3). For example, the Inter-eNB CA may use a macro cell served by a macro base station (Macro eNB (MeNB)) and a pico cell served by a pico base station (Pico eNB (PeNB)).
Further, a method has been proposed in which signals for control-plane including mobility management of a UE are transmitted and received using a macro cell having a wide coverage and data-plane signals such as user data are transmitted and received using a pico cell which provides relatively better communication quality (Non-Patent Literature 4). This method is referred to as C/U Split.